1. Field of the Invention
The present invention relates to a waveguide-type optical switch used to switch the optical transmission routes in an optical communication system.
2. Description of the Related Art
In an optical communication system, e.g., an optical loop network, the optical transmission line is doubled to improve the reliability of the optical transmission routes, and is switched by means of an optical switch at each station. A two-by-two photoswitch having 2-input/2-output function, as shown in FIG. 12, is generally known as an example of the optical switch adapted for this switching operation. This photoswitch is designed so that connectors 1 and 2 are joined end-to-end.
In this photoswitch, the connector 1 has input/output optical fibers 1a and 1b and a switching optical fiber 1c arranged at predetermined pitches, and the connector 2 has input/output optical fibers 2a and 2b and a switching optical fiber 2c arranged at the same pitches as those of the connector 1. The optical fibers 1c and 2c are optically connected to each other with their respective end portions curved like a loop. Conventionally, the photoswitch constitutes double optical transmission routes extending from the fiber 1a to the fiber 2b and from the fiber 1b to the fiber 2a.
In switching the optical transmission routes, the connector 1 is moved parallel for one pitch of the optical fibers, with respect to the connector 2, so that the transmission routes extends from the optical fiber 1a through the fibers 2c and 1c to the fiber 2a and from the fiber 1b to the fiber 2b, as shown in FIG. 13.
Since the respective end faces of its individual optical fibers are butted directly to one another, the photoswitch has an advantage over a switch which uses an optical connection system, such as a lens, for alignment in being able to effect low-loss operation despite the use of single-mode fibers.
As shown in FIGS. 14, 15 and 16, moreover, there are three states of connection, cross, through, and loop-back, between which the connection between two optical fiber pairs is switched.
In the case shown in FIG. 14, optical fiber pairs 3 and 4 are connected to each other by means of a fiber pair 5 which includes cross fibers 5a and 5b, and the optical transmission routes is cross-connected, extending from an optical fiber 3a through the cross fiber 5a to the optical fiber 4b and from an optical fiber 3b through the cross fiber 5b to an optical fiber 4a.
In the case shown in FIG. 15, on the other hand, the optical fiber pairs 3 and 4 are connected to each other by means of the fiber pair 5 which includes through fibers 5c and 5d, and the optical transmission routes are through-connected, extending from the optical fiber 3a through the through fiber 5c to the optical fiber 4a and from the optical fiber 3b through the through fiber 5d to the optical fiber 4b.
In the case shown in FIG. 16, moreover, the optical fiber pairs 3 and 4 are connected to each other by means of the fiber pair 5 which includes loop-back fibers 5e and 5f, and the optical transmission routes are loop-back-connected, extending from the optical fiber 3a through the loop-back fiber 5e to the optical fiber 3b and from the optical fiber 4a through the loop-back fiber 5f to the optical fiber 4b.
Presently, an optical switch which uses a planar lightwave circuit in place of the optical fibers is proposed as means for switching between the three states. In an optical switch shown in FIG. 17, for example, planar lightwave circuit components 6 and 7 are butt-connected to each other, and optical connectors 8 and 9 are butt-connected to the components 6 and 7, respectively. The connectors 8 and 9 are attached to the respective end portions of the optical fiber pairs 3 and 4.
In the planar lightwave circuit component 6, planar waveguides 6b, 6c, 6d and 6e are formed on a substrate 6a, and reflection mirrors M are arranged individually at bent portions of the planar waveguides 6d and 6e. Likewise, in the planar lightwave circuit component 7, planar waveguides 7b, 7c, 7d and 7e are formed on a substrate 7a, and reflection mirrors M are arranged individually at bent portions of the planar waveguides 7d and 7e.
Thus, in the cases illustrated, a light beam transmitted through the optical fiber 3a is delivered to the optical fiber 4a via the planar waveguides 6b and 7b, while a light beam transmitted through the optical fiber 3b is delivered to the optical fiber 4b via the planar waveguides 6c and 7c, whereby the optical fiber pairs 3 and 4 are through-connected.
When the optical switch is shifted in the manner shown in FIG. 18, the light beam transmitted through the optical fiber 3a is delivered to the optical fiber 4b via the planar waveguides 6b and 7c, while the light beam transmitted through the optical fiber 3b is delivered to the optical fiber 4a via the planar waveguides 6c, 7d, 6e and 7b in the order named, whereby the optical fiber pairs 3 and 4 are cross-connected.
When the optical switch is shifted in the manner shown in FIG. 19, moreover, the light beam transmitted through the optical fiber 3a is delivered to the optical fiber 3b via the planar waveguides 6b, 7d, 6e, 7e and 6c in the order named, while the light beam transmitted through the optical fiber 3b is delivered oppositely to the optical fiber 3a via the planar waveguides 6c, 7e, 6e, 7d and 6b in the order named, whereby the optical fiber pairs 3 and 4 are loop-back-connected.
In the photoswitch using the optical fibers shown in FIGS. 12 and 13, however, the fibers constituting the optical transmission routes are switched by means of the switching optical fibers 1c and 2c which are connected to each other with their respective end portions curved like a loop, so optical fibers are looped at each station.
Generally, in curving optical fibers, it is necessary to lower bending loss or prevent the breaking strength of the fibers from being lowered by bending distortion. In curving the fibers like a loop, therefore, the diameter of the curved fibers must be kept at about 60 mm or more. Thus, the photoswitch, which contains optical fibers with this diameter, is inevitably large-sized.
If the optical fibers are used to establish the three connection states, cross, through, and loop-back, as shown in FIGS. 14 to 16, the fiber arrangement is complicated, so that the cost, fiber storage spaces, etc. are not practical.
The problem of the optical fiber storage can be solved by the optical switch which uses the planar lightwave circuit shown in FIGS. 17 and 19. In this case, however, the planar lightwave circuit components 6 and 7 and the optical connectors 8 and 9, which constitute the optical switch, are butt-connected at the three regions between the connector 8 and the component 6, between the components 6 and 7, and between the component 7 and the connector 9. Inevitably, therefore, the connection loss of this optical switch is substantial, and the cost of the components of the switch is high.